It started as a routine check into a false lead on a cold case – but after five years of dogged digging, a veteran homicide detective was finally able to slap handcuffs on a suspect in the 1984 sex murder of an 11-year-old Long Island girl.

“It felt good,” Nassau County Detective Michael Kuhn beamed yesterday as authorities announced the long-awaited break in the tragic case of Massapequa schoolgirl Angela Wong, who was drowned in a creek during an aborted rape attempt.

Manuel Pacheco, 33, a personal trainer, was busted in Los Angeles and charged with the slaying. At the same time, he was also arrested for molesting one of his own daughters.

At a court hearing today, he will decide whether to waive extradition and immediately return to New York to face the murder charge.

“It has taken 17 years, eight months and five days to receive the telephone call we all longed for,” Angela’s family said yesterday. “Angela was a loving child who was viciously taken from us and has been deeply mourned by all who knew her.”

The petite sixth-grader vanished one July evening after leaving her home for a disco party. Her partially clothed, battered body was found in a shallow stream in the woods two blocks from her home the next day.

Despite a reward and much publicity, the case went unsolved. When Kuhn dusted off the files to run down a lead five years ago, it didn’t pan out – but the tragedy piqued his interest.

Often working in his free time, the 32-year veteran began re-interviewing witnesses and family, including Angela’s father, Angelo, who now publishes books on bereavement in Florida.

“Neither one of us ever gave up hope,” Kuhn said.

Eventually, Kuhn tracked down a woman in Virginia who had seen Angela walk into the woods with Pacheco, then 15, and never told cops about it.

Then Kuhn and his boss, Detective Sgt. William Cocks, headed to Los Angeles – Pacheco’s home since 1990 – and found several women who claimed the suspect told them “he had, in fact, done this murder,” Cocks said.

Pacheco, a father of six, had “bizarre” relationships with three different women in L.A. – all of whom knew about the others and were “terrified” of him, Cocks said.

When he was arrested March 20, he was driving a $50,000 car registered to his mother, a Bronx resident, even though his only “visible means of support” was his job as a health-club trainer, police said.

Pachecho was charged as an adult but if he’s convicted, he will only face 15 to 25 years in prison because he was a minor at the time of the crime.

When he was arrested, Pacheco was also booked for allegedly molesting one of his daughters. He has hired former O.J. Simpson prosecutor Christopher Darden to defend him. Darden did not return phone calls.